The invention relates to a tachometer of the type which comprises a rotation pick up shaped as a phonic wheel, that is, a wheel having teeth of ferromagnetic material and which is connected to or forms part of the turning body which is the object that is to be measured tachometrically and which rotates adjacent a magnetic transducer. The frequency of the alternating current which is delivered by the transducer obviously is proportional to the rotation speed of the wheel as well as, in certain cases, the amplitude of the delivered voltage. Thus, the two electrical parameters of the rotation speed are determinable. Tachometers are already known which comprise phonic wheels which do measure the one or the other of these parameters.
Certain tachometers of the above-cited type of a first category measure the amplitude of the voltage from the transducer and the speed is indicated by a galvanometer which is graduated in units of rotation speed or frequency. These devices have the advantage that they do not need a source of current supply; yet their measurements contain errors which are due to the fact that the geometry of the pick up is not stable, variations of the air gap between the teeth and the magnetic pole, e.g. which are due to changes in the operating temperature and to structural deformations and which can only be compensated for by supplementary circuits which complicate the tachometric measurements.
Tachometers of the above-cited type of a secondary category which do measure the frequency comprise a frequency meter as a speed indicator. No doubt these devices are more precise and accurate than those of the previous category but those which are known require an external or autonomous source of current to feed the frequency meter.
As examples of prior devices see U.S. Pat. No. 3,826,985 and British Pat. No. 855,611.